There presently exist television projection systems that utilize a type of semiconductor device known as a Digital Micromirror Device (DMD). A typical DMD comprises a plurality of individually movable micromirrors arranged in a rectangular array. Each micromirror pivots about limited arc, typically on the order of 10°-12° under the control of a corresponding driver cell that latches a bit therein. Upon the application of a previously latched “1” bit, the driver cell causes its associated micromirror cell to pivot to a first position. Conversely, the application of a previously latched “0” bit to the driver cell causes the driver cell to pivot its associated micromirror to a second position. By appropriately positioning the DMD between a light source and a projection lens, each individual micromirror of the DMD device, when pivoted by its corresponding driver cell to the first position, will reflect light from the light source through the lens and onto a display screen to illuminate an individual picture element (pixel) in the display. When pivoted to its second position, each micromirror reflects light away from the display screen, causing the corresponding pixel to appear dark. An example of such DMD device is the DMD of the DLP™ projection system available from Texas Instruments, Dallas Tex.
Present day television projection systems that incorporate a DMD of the type described control the brightness (illumination) of the individual pixels by controlling the duty cycle during which the individual micromirrors remain “on” (i.e., pivoted to their first position), versus the interval during which the micromirrors remain “off” (i.e. pivoted to their second position). To that end, such present day DMD-type projection systems use pulse width modulation to control the pixel brightness by varying the duty cycle of each micromirror in accordance with the state of the pulses in a sequence of pulse width segments. Each pulse width segment comprises a string of pulses of different time duration. The actuation state of each pulse in a pulse width segment (i.e., whether each pulse is turned on or off) determines whether the micromirror remains on or off for the duration of that pulse. In other words, the larger the sum of the widths of the pulses in a pulse width segment that are turned on (actuated), the longer the duty cycle of each micromirror.
In a television projection system utilizing a DMD, the frame interval, i.e., the time between displaying successive images, depends on the selected television standard. The NTSC standard currently in use in the United States requires a frame interval of 1/60 second whereas certain European television standards employ a frame interval of 1/50 second. Present day DMD-type television projection systems typically provide a color display by projecting red, green, and blue images either simultaneously or in sequence during each frame interval. A typical sequential DMD-type projection system utilizes a motor-driven color wheel interposed in the light path of the DMD. The color wheel has a plurality of separate primary color windows, typically red, green and blue, so that during successive intervals, red, green, and blue light, respectively, falls on the DMD.
To achieve a color picture, red, green and blue light must fall on the DMD at least once within each successive frame interval. If only one red, one green and one blue image is made and each consumes ⅓ of the frame interval, then the large time interval between colors will produce perceptible color breakup with motion. Present day DMD systems address this problem by breaking each color into several intervals and interleaving the intervals in time, thereby reducing the delay between colors. Each color interval corresponds to a pulse width segment, with the pulse width segment for each color interleaved with the segments of the other colors.
In addition to color break up with motion, another type of motion artifact can appear in a DMD-type television projection pulse width modulated display system. The particular color that appears in the first segment can adversely affect the appearance of moving white object. For example, making the first segment green will produce a green leading edge for the white moving object. For similar reasons, making the last segment green is also undesirable. Ideally, the color green should appear (in terms of brightness) during the middle of the interval of the sequence of pulse width segments.
Thus there is a need for a technique for operating a pulse width modulated sequential display that reduces motion artifacts.